1. Field of the Invention
This invention relates to the denitrification of water and, in particular, relates to a process for the biological denitrification of agricultural drain waters, sewage effluents, ground waters and the like.
2. Brief Statement of the Prior Art
The intensive use of nitrogenous fertilizers to achieve spectacular gains in agricultural production has resulted in the run-off of substantial quantities of nitrogen in the form of nitrates to surrounding rivers, lakes and oceans. The problem encountered with dissolved nitrogen compounds in all wastewaters is particularly acute in irrigated lands in arid and semiarid regions of the country. Irrigated lands must, periodically, be flushed with large quantities of water to remove salts that otherwise accumulate in the soil. The resultant agricultural drain waters, which are collected in subterranean drainage channels, must be separately collected and processed since their relatively high content of dissolved salts, and in particular, nitrates, renders them unfit for returning to the surrounding watershed.
Sewage effluents are also high in nitrogen compounds which are readily converted into nitrates which when discharged into streams, lakes and ground water basins makes them unfit for other beneficial uses. Additionally, some groundwaters are already contaminated with nitrates and require some form of treatment before use.
Various techniques have, in the past years, been proposed for the removal or reduction of the nitrates in waters. Some methods have proposed the use of ion exchange resins. While ion exchange resins are operative for this purpose, their high initial and operational costs renders this technique unsuited for an ecconomical solution of the problem.
Separate proposals have been made for the biological reduction of the nitrate content of water by the use of anaerobic bacteria. Since the waste waters commonly contain a relatively low organic content, typically less than about 5 parts per million, these waters have insufficient energy sources to support growth of adequate quantities of anaerobic bacteria for any substantial reduction in the nitrate content of the water. Accordingly, various materials have been suggested as suitable to add to the waters for support of the growth of the anaerobic bacteria. Among such suggestions are liquids such as methanol or acetic acid. Another suggestion, in U.S. Pat. No. 3,829,377, is to add methane or a gaseous hydrocarbon as the food source for the bacteria. In this method a gaseous hydrocarbon such as methane is continuously passed through the water to maintain the water saturated with the gas during its residence in the bacterial zone.
The aforedescribed techniques are not ideally suited to the most ecconomical and practical solution of the problem for various reasons. The use of miscible liquids such as methanol or acetic acid presents a residual purification problem since excess quantities of the energy source will remain in the treated water, resulting in a polution problem which may be as severe as the nitrate contaminants desired to be removed. Additionally, these liquids are industrial chemicals in demand for other purposes and, hence, costly for use in the denitrification process. Substitution of methane for methanol or acetic acid does not entirely obviate these problems. While the use of a gaseous material does not present a potential contamination problem, the gases must be continuously recycled through the water to maintain its saturation during treatment, thereby requiring expenditure of substantial amounts of energy, in addition to the energy source, such as methane, consumed by the bacteria.
Ideally, the denitrification process should utilize, as the food source for anaerobic bacteria, a material which does not impose a burdensome demand for expensive industrial chemicals and energy sources. In some instances, the process could find commercial applications by using as the biological energy source a material which is a waste product of the society and disposal of such material is achieved concurrently with the restoration of waste waters to ecologically acceptable standards. Such a technique would also avoid consumption of energy sources such as methane or petrochemicals such as methanol and acidic acid which are already experiencing excessive demand in an energy limited society.